Tissue may be destroyed, ablated, or otherwise treated using thermal energy during various therapeutic procedures. Many forms of thermal energy may be imparted to tissue, such as radio frequency electrical energy, microwave electromagnetic energy, laser energy, acoustic energy, or thermal conduction. In particular, radio frequency ablation (RFA) may be used to treat patients with tissue anomalies, such as liver anomalies and many primary cancers, such as cancers of the stomach, bowel, pancreas, kidney and lung. RFA treatment involves destroying undesirable cells by generating heat through agitation caused by the application of alternating electrical current (radio frequency energy) through the tissue.
Various RF ablation devices have been suggested for this purpose. For example, U.S. Pat. No. 5,855,576 describes an ablation apparatus that includes a plurality of electrode tines deployable from a cannula. Each of the tines includes a proximal end that is coupled to a generator, and a distal end that may project from a distal end of the cannula. The tines are arranged in an array with the distal ends located generally radially and uniformly spaced apart from the distal end of the cannula. The tines may be energized in a bipolar mode (i.e., current flows between closely spaced electrode tines) or a monopolar mode (i.e., current flows between one or more electrode tines and a larger, remotely located common electrode) to heat and necrose tissue within a precisely defined volumetric region of target tissue. To assure that the target tissue is adequately treated and/or to limit damaging adjacent healthy tissues, the array of tines may be arranged uniformly, e.g., substantially evenly and symmetrically spaced-apart so that heat is generated uniformly within the desired target tissue volume.
When using the above described devices in percutaneous interventions, the cannula is generally inserted through a patient's skin, and the tines are deployed out of the distal end of the cannula to penetrate target tissue. Particularly, the tines are deployed such that the distal ends of the tines initially exit from a distal opening at the cannula. As the tines are further deployed, the distal ends of the tines evert radially away from an axis of the cannula, and then back towards a proximal end of the cannula (so that they face substantially in the proximal direction when fully deployed). As such, the tines/electrodes of the above described device each has a profile that resembles a parabola after the electrodes are deployed. The tines are then energized to ablate the target tissue.
It has been found that deployed electrodes having parabolic profiles have relatively low column strength, thereby allowing the electrodes to easily buckle. The buckling of the electrodes may occur within the cannula as the electrodes are being advanced within the cannula. In other cases, the buckling of the electrodes may occur outside the cannula as the electrodes penetrate through tissue (e.g., dense tissue). This is especially true with ablation probes that are used to create large size lesions. In such cases, longer wires are used in order to create longer tines, such that the array of tines will span across tissue have a certain size (e.g., cross-sectional area/dimension) when the tines are deployed. Since a column strength of a tine is inversely proportional to the length of the tine, creating tines using long wires will cause the tines to have low column strength. In some cases, the cross-sectional size of a tine can be increased to improve the tine's column strength. However, increasing the cross-sectional size of the tines increases the overall size of the ablation probe, making the ablation probe less desirable for treatment.
Ablation devices having a flat electrode array have been described in U.S. patent application Ser. No. 10/668,995. In such devices, the electrodes have a sharp 90° bent followed by a substantially flat/straight profile, such that the electrodes extend in directions that are substantially perpendicular to a longitudinal axis of the cannula when deployed from the cannula. Such configuration is particularly beneficial for generating flat lesions. However, in some cases, it may be desirable to generate lesions that are relatively more voluminous. Also, electrodes having the above configuration may undergo excessive bending stress (because of the sharp 90° bent) when housed within a cannula, and may be difficult to be deployed from the cannula.
Thus, there remains a need to provide for improved ablation devices having electrodes with good column strength. There is also a need to provide for improved electrodes that can be housed within a cannula without inducing excessive stress on the electrodes.
Another problem associated with existing ablation devices is that they tend to create lesions that are symmetrical. For example, another existing ablation device includes two electrode arrays that are spaced from each other, wherein the arrays have the same configuration (e.g., same deployed profile and same number of electrodes). Such ablation devices create lesions that are substantially symmetrical. However, in some cases, it may be desirable to create lesions that are asymmetric, or lesions that have other customized shapes.